Add configurable boot grace time

This guards against the case when the RX takes longer to obtain a valid
link than the gyro takes to calibrate (previously this delay is all that
stopped immediate arming).

src/main/fc/fc_init.c

@@ -719,6 +719,8 @@ void init(void)
     latchActiveFeatures();
     pwmEnableMotors();
 
+    setArmingDisabled(ARMING_DISABLED_BOOT_GRACE_TIME);
+
 #ifdef USE_OSD_SLAVE
     osdSlaveTasksInit();
 #else